Variable capacitors may be used for automatic circuit tuning. In low power circuits, for example, a varactor, which is a semiconductor device having a junction capacitance that varies with bias voltage, is often used for automatic tuning. For high power, high cost applications, mechanical actuators, e.g., stepper motors, with feedback may be used to control capacitance. Unfortunately, neither of these approaches is suitable for electrodeless high intensity discharge (HID) lamp ballasts, which are typically high power, low cost applications.
In an electrodeless HID lamp, an arc discharge is generated by establishing a solenoidal electric field in a gas contained within an arc tube. The solenoidal electric field is created by the time-varying magnetic field of an excitation coil which is disposed about the arc tube. To maximize efficiency of an HID lamp, the degree of coil coupling between the magnetic field and the arc discharge must be maximized. Since the degree of coupling increases with frequency, electronic ballasts used to drive HID lamps operate at high frequencies in the range from 0.1 to 30 MHz, exemplary operating frequencies being 13.56 and 6.78 MHz. These exemplary frequencies are within the industrial, scientific, and medical (ISM) band of the electromagnetic spectrum in which moderate amounts of electromagnetic radiation are permissible; and such radiation is generally emitted by an electrodeless HID lamp system.
Operation of an HID lamp ballast at the series resonant frequency of the load circuit maximizes power output. However, operation at a frequency slightly higher than the series resonant frequency of the load circuit maximizes ballast efficiency. Hence, for maximum efficiency, operation is slightly "off" resonance, and a specific ballast load resistance and phase angle are required. To this end, the impedance of the ballast load, including that of the arc discharge as reflected into the ballast load, must be matched to the required ballast load resistance and phase angle. As described in commonly assigned, U.S. Pat. No. 5,047,692 of J. C. Borowiec and S-A El-Hamamsy, issued Sept. 10, 1991, which is incorporated by reference herein, a capacitance connected in parallel with the excitation coil is needed to match the resistive component of the ballast load impedance, and a capacitance connected in series with the excitation coil is needed to obtain the proper phase angle. However, the ballast load impedance, and thus the matching conditions, for running and starting the lamp are different. In addition, the circuit is very sensitive to component variations because its quality factor (Q), when the lamp is running, is very high, e.g., 20-40.
Accordingly, it is desirable to provide an automatically variable capacitor useful for tuning an electrodeless high intensity discharge lamp ballast as the output impedance thereof changes between starting and running conditions. In addition, it is desirable that such a variable capacitor be automatically tunable for matching the ballast impedance as it changes due to impedance variations in circuit components.